If you’ve been thinking of developing your first custom ROM for your Sony Xperia device, chances are that you probably need a kernel to go along with it, especially if the ROM is for devices with locked bootloaders. Because of this, it’s sometimes best to know how to extract the kernel from an FTF file in order to integrate it into your ROM.
A great place to begin with would be a tutorial by XDA Recognized Contributor matt4321 and XDA Recognized Contributor and Themer DaRk-L0rD. Divided into two parts, the tutorial first guides you through the steps of extracting a kernel from an FTF file and converting it into a boot.img file. The second part teaches you how to make a flashable ZIP package containing your freshly extracted boot.img file.
Additionally, because the kernel is located in different partitions in different Xperia devices, DaRk-L0rD also includes a handy list of just where you can find the kernel for your particular Xperia device. Just keep in mind, however, that not all Xperia devices are on the list at this time, so DaRk-L0rD also provides a simple way of finding out the directory yourself.
If you would like to find out more, check out the original thread for more information.